1. Field of the Invention
This invention relates to an automatic focusing system advantageously adapted for a video apparatus such as a video camera, electronic still camera or the like.
2. Description of the Related Art
Various methods have hitherto been developed for the automatic focusing systems of cameras. In the case of an apparatus of the kind having image sensing means for obtaining a video signal by photo-electrically converting an object image, such as a video camera or an electronic still camera, it is known that the automatic focusing system is arranged to detect the sharpness or definition of the object image from the video signal and to control the position of a focusing lens in such a way as to obtain a maximum degree of image sharpness.
Meanwhile, a method for controlling a speed at which the focusing lens is to be driven by the focusing system of the above-stated kind has been disclosed, for example, in U.S. patent application Ser. No. 500,297 filed on Mar. 27, 1990. In accordance with this method, the blur width of the edge of an object image is detected and the focusing lens driving speed is adjusted according to the detected value of the blur width.
In the case of the method for detecting focus by extracting from the video signal a signal component which varies with the state of focus, it is sometimes difficult to accurately make focus adjustment in a natural manner as the signal component fluctuates according to the object and the environment thereof.
The control operation of the focusing system therefore includes some ambiguities in itself. It may appear that a fuzzy inference which has recently begun to be used for control in various fields is effectively applicable to focusing adjustment control. It is conceivable to apply the fuzzy inference to the focusing system of this kind, for example, in the following manner: the high-frequency component of the video signal and the detected value of the blur width of the object image are processed by the fuzzy inference. Then, the automatic focusing action can be accomplished by determining the speed and direction of a focusing lens driving action on the basis of the result of the fuzzy inference process.
In using the fuzzy inference for focusing lens driving control, membership functions are defined for the detected value of the blur width of the object image and the value of the high-frequency component of the video signal. In actuality, however, both the detected value of blur width and that of the high-frequency component greatly fluctuate with the conditions of the object. The degree of such fluctuations might exceed too much the limit of the intrinsic ambiguities of the membership functions. In such a case, the focusing motor driving control cannot be adequately accomplished by the fuzzy inference.
More specifically, in the case of the automatic focusing system of this kind, the detected value of blur width of the object image and the signal value of the high-frequency component become lower than usual at An in-focus point in the event of an object of low contrast. In that event, if the membership functions are set in a normal manner, they would be judged to be lower than the actual values. Then, the driving speed of a focusing motor would be set at a high speed even in the neighbor hood of an in-focus point and the focusing lens cannot be brought to a stop accurately at the in-focus point. Further, in the event of a high-luminance object, the conditions of the membership functions might be judged to be higher than the actual values. The driving speed of the focusing motor is then likely to be set at a speed too low. In that case, either the focusing motor comes to a stop before reaching an in-focus point or an excessively long period of time becomes necessary for focusing.